The spell mirage arcane is problematic in that much is left undefined, or unclear. It is a 7th level spell, so its powers are unexpectedly great. Nonetheless, there must be some limitations to the spell. I’m specifically interested in the type of terrain that the local one can be changed to.
Jeremy Crawford once unofficially tweeted that targets could drown in an imaginary lake, or fall off an imaginary cliff, so we can assume that spawning a lake in the middle of nowhere isn’t too far-fetched.
The mirage arcane spell gives you tremendous latitude in how you make the affected terrain look and feel. The altered terrain can even hurt someone. You could drown in the spell’s illusory lake, for example, or fall off an illusory cliff.
1. Can extraplanar terrain be used? Places like the Elemental Plane of Fire are inherently dangerous to humans. Could you use Mirage Arcane to summon typical terrain from that plane, like a sea of fire (for example)?
2. Do acid lakes, toxic bogs, and active volcano calderas count as terrain? I’m inclined to say that if one can drown in an illusory lake, then surely one can melt in an illusory acid lake. The toxic bog though, it’s more debatable if its terrain components are the deadly ones. And I’m not sure if a big lake of lava is terrain.
3. Does the allowed terrain depend on location? Perhaps summoning a lake of lava in the middle of a calm meadow is a bit extreme (then again, perhaps not for a 7th level spell). However, surely if one were to use this spell in a volcanic landscape, then it would be appropriate?